In view of the growing demands of the automotive industry, new requirements have emerged directly reflected in a greater stress on the components of internal combustion engines. In this manner, the current engine components, not being designed for such demand, experience premature wear. Some of the parts which naturally experience this effect are the sliding elements, particularly piston rings and bearings.
Innumerable developments have emerged with the objective of improving the fatigue strength and the wear resistance of the components of an internal combustion engine, nevertheless the increase in the working pressures of the internal combustion engines and the growing demand for reduction in friction and increase in the durability of the components of an engine render the success of such attempts difficult by virtue of their leading on one of the sliding elements, or on the other components of the engine, to premature wear and failure, in addition to not satisfying the new demands such as the reduction in friction having direct impact on the reduction of emissions.
It is also noted that the future will not be straightforward for this field of activity, it being sufficient to observe that more potent engines are required attaining higher rotational speeds, efficiently, with lower consumption and with high load capacity. All these factors threaten the development of the engine in the long term, prejudicing the functioning of the engine, or moreover the failure thereof.
Naturally, in view of the new requirements, the emergence of new materials is required given that those already known are the same ones restricting the development of internal combustion engines today. Some of the most important components in order to achieve a better engine performance are the sliding elements, such as piston rings, bearings, pins, etc. Due to the innovations in sliding elements, having greater hardness, greater fatigue strength and wear resistance, low coefficient of friction, and consequently longer working life, the automotive industry has modernised in tandem with the development of increasingly efficient, powerful and durable engines, confronting the high load to which they are subjected.
It is understood that the greater the hardness of the coating applied to the sliding element the lower the rate of wear, however the greater the hardness of the sliding element the more fragile it becomes and more internal stresses are generated during the manufacture thereof, resulting particularly in an increase in the probability of delamination of the coating on the sliding element, detachment of the hardest layer coating the component, or even the breakage or rupture of the component.
In this sense, sliding elements of the state of the art have attempted to reach an equilibrium through having a coating of sufficiently high hardness such that it does not wear easily in contact with other components of the engine. Such an effect has been achieved through deposition of films of DLC (diamond-like carbon), the electronic structure of the carbon whereof is principally sp3. More particularly, this type of deposition makes use of tetragonal amorphous carbon (ta-C), that is to say carbon free of or having low hydrogen content.
As a rule, upon the metal base of a sliding element there is applied a connecting layer which subsequently receives the layer of DLC, it being possible that the metal base receives a nitriding treatment or a PVD coating prior to receiving the connecting layer.
In fact, films deposited with DLC tend to have a very great wear resistance by virtue of the high hardness thereof, however they also present disadvantages. It is not solely the hardness which ensures the longevity of a component coated with DLC, but also the thickness of the film. In this sense, at least in theory, thicker films offer greater durability for the same rate of wear. Nevertheless, as is known, the deposition of a film of DLC generates very high internal stresses, creating limitations in the process of deposition and growth of thicker layers.
One of the documents of the state of the art relevant to the present invention is the U.S. Pat. No. 8,123,227 describing a sliding element wherein the sliding surface is coated with a hard film of amorphous carbon having a roughness equal to or less than 0.7 micrometers and an initial wear height lying between 0.07 and 0.14 micrometers.
For a film or a coating of pure carbon the high hardness is a critical factor for the obtainment of a suitable surface finish, fundamental for the low wear on the mating body, an engine liner. For this reason, in this coating, the roughness needs to be controlled with the objective of minimising the initial wear, generating additional costs in the process and rendering the coating unviable for some automotive applications. The layers described in this coating are capable of being finished, however due to the high internal stress the thickness is limited to between 2 and 4 microns. In the proposed coating, the tungsten functions as an internal stress reducer, rendering possible the construction of layers having a thickness exceeding 10 microns, without prejudicing the wear resistance thereof.
The document EP 2432913 discloses a sliding element for internal combustion engines, especially piston rings, provided with a coating of DLC of the ta-C type having at least one residual stress gradient. In this case, to obtain thick coatings manipulation of the internal stresses is utilised by means of the manipulation of the sp2 and sp3 structures. Furthermore, it is known that coatings having sp2 structures have less wear resistance.
The document JP 2013528697 also describes a sliding element and a process for obtaining a sliding element for internal combustion engines, especially piston rings, being provided with at least one sliding surface having a coating comprising, from inside to outside, metal provided with an adhesive layer and a layer of DLC of the ta-C type having a thickness of a minimum of 10 microns. To obtain thick coatings the manipulation of the internal stresses is utilised, permitting up to 60% of the sp2 structure, and there is also contemplated the addition of hydrogen; both cases led to a reduction in the wear resistance, by virtue of the coating being distant from the pure columnar structure of diamond (100% sp3).
The document EP 2574685 describes a sliding element and method of obtainment of the sliding element comprising a coating of the diamond-like carbon (DLC) type upon a substrate of the sliding element, and a material 20% to 40% softer than the DLC deposited on the surface of the coating of DLC, wherein the aforementioned softer material comprises a metal or metal oxide softer than the coating of DLC. The solution favours the matter of conditioning/running in of the surface at the commencement of operation, adding a soft material however having a higher coefficient of friction, leading to the loss of performance of the engine at the commencement of operation thereof.
The Japanese document JP 4331292 discloses a composite film provided with good adhesion to the base material, together with a low coefficient of friction due to the formation of a first hydrogen-free layer which is deposited upon the base, and the formation of a second layer including hydrogen which acts as the sliding layer.
This coating is a matter of a system having at least two layers, wherein one of the layers is hydrogenated, it being known that hydrogenated layers have a higher coefficient of friction and lower wear resistance when compared with hydrogen-free layers of DLC. The present invention is hydrogen-free.
Consequently, in addition to the limitation on the increase of the thickness of the layers, there is a limitation in respect of the homogeneity of the hardness in the coated layer. This lack of uniformity flows from the variation of the residual stresses which, during the deposition of the DLC, need to be monitored, reducing the intensity of the deposition to maximise the prevention of the accumulation of internal stresses in the most susceptible regions. The result is a film having a variation in hardness, leading to prejudicing the wear resistance and reducing the working life of the component.
In this manner, one of the great problems existing today in the deposition of ta-C films arises from the lack of uniformity of the hardness and of the inability to reduce the internal stresses, preventing the growth of layers of DLC having greater thickness.
It is consequently necessary to achieve a solution ensuring the matter of durability, heeding the necessity of wear not occurring on the components with which the sliding element interacts and, moreover, of maintaining a homogenous hardness throughout the surface of the film, concomitantly rendering it possible to deposit the film of DLC with less internal stress.